As difficult as Zero Dark Thirty’s much-discussed interrogation scenes are to watch, they were much harder to film. Jessica Chastain, who stars as a tough-minded C.I.A. agent named Maya who’s hunting Osama bin Laden, says that those sequences – which feature waterboarding and other rough tactics – were so emotionally grueling that at one point she had to take a break to go cry behind a building. “It was all super-tough to shoot, but the interrogation scenes were the toughest, because I don’t like confrontation and we were filming in an active Jordanian prison on the outskirts of Amman,” Chastain writes in an essay in EW’s upcoming Best & Worst issue.

“Imagine going to work every day and there are barricades and guards. You can’t take a cellphone, you have to be searched before you go in, and there were certain times of the day I was not allowed to be outside.”

But the hardest part was not being able to channel the stress of that uncomfortable environment into her performance. “It was rough, and I couldn’t purge my feelings through my character,” she writes. “I couldn’t be in the scene and have a good cry because Maya couldn’t do that. I’m playing a character who’s trained to be unemotional and analytical, but I’ve been trained to let the walls down and open my heart and be vulnerable.”

Zero Dark Thirty opens Dec. 19 in select theaters.

For more from Chastain (along our picks for the best and worst movies, TV shows, albums, books, and more), pick up EW’s Best & Worst issue, on newsstands Dec. 21.